The present invention relates to an air-fuel ratio control method for controlling, by adjusting the fuel feeding rate, the air-fuel ratio condition of an internal combustion engine at a desired condition which is different from a stoichiometric condition.
There is known an internal combustion engine which controls its air-fuel ratio condition at a desired condition on the lean side with respect to a stoichiometric condition, by intermittently injecting fuel from at least one electric fuel injection valve. In such an engine, the closed loop air-fuel ratio control for controlling the air-fuel ratio condition depending upon a signal from the exhaust gas sensor, which detects the concentration of a certain component, such as the oxygen component, contained in the exhaust gas, cannot be executed. This is because the existing exhaust gas sensor (hereinafter called an O.sub.2 sensor) only discriminates whether the air-fuel ratio condition surrounding the sensor is on the rich side or on lean side with respect to the stoichiometric condition. In other words, the existing O.sub.2 sensor cannot discriminate whether or not the condition surrounding the sensor becomes a desired condition, which is on lean side with respect to the stoichiometric condition.
Therefore, a lean burn engine, in which the air-fuel ratio condition is controlled at a lean condition, has to control the air-fuel ratio condition by an open loop control operation without using the O.sub.2 sensor. Namely, in the lean burn engine, the fuel feeding rate is adjusted, depending upon its intake air flow rate or its intake manifold vacuum pressure and upon its rotational speed. No signal from the O.sub.2 sensor is used. According to such an open loop control, it is difficult to automatically compensate not only the amount of scatter, or error, measured by the sensors for detecting the engine parameters, for example, the air-flow sensor, the manifold vacuum pressure sensor, the rotational speed sensor and the like, but also the amount of scatter in the controlled fuel rate by the fuel injection valve, of each engine. As a result, the controlled air-fuel ratio condition of each engine, although each one has the same type sensors and injection valve, becomes different from each other. Particularly, in the lean burn engine, the scatter in the controlled air-fuel ratio condition causes the characteristics of the emitted amount of HC, CO and NO.sub.x from the engine fuel consumption, and engine torque to extremely deteriorate.